In FIG. 1, there is shown a conventional linear-type multi-head sewing machine, which includes a machine table 10, a machine frame 11 upward extended from the machine table 10, a plurality of sewing heads 12 mounted on the machine frame 11 to space from one another, a plurality of needle plates 13 disposed on the machine table 10 at positions opposite to the respective sewing heads 12 and having an extending direction perpendicular to that of the sewing heads 12, a plurality of rotating shuttles (not shown) disposed below respective needle plates 13 to cooperate with the sewing heads for sewing operation, a substantially rectangular supporting frame 14 provided on a top of the machine table 10 for supporting a workpiece thereon, and a driving mechanism (not shown) coupled to the supporting frame 14 for driving the supporting frame 14 to horizontally move on the top of the machine table 10 in the x-direction and the y-direction. On each of the sewing heads 12, a plurality of needle bars 15 is linearly arranged to supply different types of threads. Therefore, any desired one of the needle bars 15 can be selected for stitching with a desired type of thread.
However, according to the above-described multi-head sewing machine, the needle bars 15 on each sewing head 12 are usually linearly arranged, and every needle bar 15 has a fixed lowering position. Therefore, each needle plate 13 for feeding the workpiece on the machine table 10 must have a size large enough to cover an overall width of all the needle bars linearly arrayed on the corresponding sewing head 12 as well as an overall width of a stitching area on the workpiece. As a result, the supporting frame 14 on the machine table 10 would be very large in volume. Moreover, in practical sewing operation, in case there is a large number of needle bars 15 provided on each of the sewing heads 12, it is necessary to increase the driving mechanism's travel distance in the x-direction, so that the same point of the workpiece on the machine table 10 can be moved to the positions corresponding to the leftmost and the rightmost needle bar 15 on the sewing head 12 for stitching.
To reduce the driving mechanism's travel distance in the x-direction, a conventional round-type multi-head sewing machine as shown in FIG. 2 has been developed. The round-type multi-head sewing machine similarly includes a machine table 16, a supporting frame 17 upward extended from the machine table 16 for supporting a plurality of spaced sewing heads 18 thereon, and a plurality of needle bars 19 arrayed on each of the sewing heads 18. Unlike the conventional linear-type multi-head sewing machine, the needle bars 19 in the round-type multi-head sewing machine are circumferentially arranged on each of the sewing heads 18 for supplying different threads. While the circumferential arrangement of the needle bars 19 on each of the sewing heads 18 indeed reduces the x-direction travel distance, every needle bar 19 on the sewing heads 18 still has a fixed lowering position. In case there is a large number of needle bars 19 provided on each of the sewing heads 18, an increased x-direction travel distance on the machine table 16 is still needed.
In view that the conventional multi-head sewing machines have the drawbacks of having a very large volume and requiring an extended x-direction travel distance for the driving mechanism, it is therefore tried by the inventor to develop an improved sewing machine needle bar changing device in order to overcome the drawbacks in the conventional multi-head sewing machines.